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Effects of fatigue duration and muscle type
on voluntary and evoked contractile properties
D. G. BEHM AND D. M. M. ST-PIERRE
School of Physical and Occupational Therapy, McGill University,
Montreal, Quebec, Canada H3G 1Y5
Behm, D. G., and D. M. M. St-Pierre. Effects of fatigue
duration and muscle type on voluntary and evoked contrac-
tile properties. J. Appl. Physiol. 82(5): 1654–1661, 1997.—
The effects of fatigue duration and muscle type on voluntary
and evoked contractile properties were investigated with an
isometric, intermittent, submaximal fatigue protocol. Four
groups performed contractions of the plantar flexors and
quadriceps at various intensities to produce long (LDF; 19
min 30 s)- and short-duration fatigue (SDF; 4 min 17 s). The
LDF group had a significantly greater decrease in muscle
activation than did the SDF group (12 vs. 5.8%) during
recovery, although there was no difference in the impairment
of maximum voluntary contraction force beyond 30 s of
recovery. The significant decrease in the compound muscle
action potential of the LDF group (M-wave amplitude; 14.7%)
contrasted with the M-wave potentiation of the SDF group
(15.7%), suggesting changes in membrane excitation may
affect LDF. The quadriceps group performing contractions at
50% MVC experienced a smaller decrease in agonist electro-
myograph activity than did other groups, indicating both
muscle and fatigue duration specificity. Impairments in exci-
tation-contraction coupling were indicated by changes in
quadriceps peak twitch and time to peak twitch while de-
creases in PF M-wave amplitudes suggested a disruption of
membrane potentials. Results suggest that fatigue mecha-
nisms may be duration (activation, half relaxation time) or
muscle specific (electromyograph, twitch torque) or a combina-
tion of both (M wave, time to peak twitch torque).
recovery; muscle activation; twitch; electromyography; M wave
FATIGUE STUDIES have demonstrated a diversity of
mechanisms underlying fatigue-associated decrements
in force (for review see Refs. 1, 14, 16, 21). These
mechanisms are commonly subdivided in central (a-
motoneuron pool or above) as well as distal sites
(motoneuron end plate and below) and may be task
dependent (14). Indeed, changes induced by fatigue
may well be influenced by whether the muscle is
contracting voluntarily or is being induced to contract
(27) and by whether the contraction is static (5–7, 22)
or dynamic (11), sustained vs. intermittent (12), maxi-
mal (5–7) or submaximal (17, 24), or dependent on the
characteristics of the specific muscle.
Muscles with higher percentages of fast-twitch fibers
have been shown to fatigue more rapidly than do
muscles with a greater percentage of slow-twitch fibers
(4, 11, 23). Furthermore, similar fatigue protocols in a
variety of muscles have resulted in dissimilar changes
in force (10, 25); muscle activation (4), as measured
with the interpolated twitch technique (ITT); electro-
myograph (EMG) activity (24, 27); and M wave (26).
This suggests that mechanisms underlying fatiguemay
differ depending on the muscle (6) or its fiber composi-
tion (11, 23). However, because the time to fatigue
differs in different muscles, it is difficult to determine
whether the differences in underlying fatigue mecha-
nisms are muscle or duration dependent. More specifi-
cally, it is not known whether the mechanisms underly-
ing fatigue would be the same in two muscles of
different fiber type composition (20) if the time to
fatigue were similar. To compare the influence of simi-
lar fatigue durations on two different muscles of differ-
ent fiber type composition [quadriceps and plantar
flexors (PF)], different contraction intensities were
utilized to alter the duration of an intermittent, sub-
maximal, isometric, fatigue protocol.
EXPERIMENTAL DESIGN AND METHODOLOGY
Subjects. The study had four groups of eight moderately
active to active subjects (Table 1). Subjects were recruited
from the McGill University staff and student population,
were fully informed of the procedures, and signed a consent
form before experimentation. The study was approved by
McGill University’s Ethics Committee.
Experimental setup. Subjects were seated in a straight-
back chair with hips and knees at 90°. PF subjects had their
legs secured in a modified boot apparatus with their ankles at
90° (2). Quadriceps subjects were seated in a Cybex chair
(Lumex, NY) with their feet in a padded strap attached to a
strain gauge, perpendicular to the lower limb. All voluntary
and evoked torques were detected by a force transducer (PF:
custom design; quadriceps: BLH Electronics 3SB), amplified
(model NL107, recording amplifier and model NL106 analog-
to-digital converter differential amplifiers, Neurolog Sys-
tems), and monitored on an oscilloscope (model 2220, Tektro-
nix). All data were stored on computer (Seanix ASI 9000, 486
DX) at a sampling rate of 2,000 Hz after being directed
through an analog-to-digital board (Lab Master). Data were
recorded and analyzed with a commercially designed soft-
ware program (Actran, Distributions Physiomonitor, Mon-
treal, PQ).
Bipolar surface-stimulating electrodes were secured to the
superior and distal aspects of the triceps surae or quadriceps
muscle group. Stimulating electrodes were constructed in the
laboratory from tin foil coated with a conduction gel, wrapped
in cheesecloth and paper, and immersed in a saline solution.
The electrode length was sufficient to wrap the width of the
muscle belly with an electrode width of ,4–5 cm. The
electrodes were placed in approximately the same positions
for each subject. Surface EMG-recording electrodes were
placed 3–5 cm apart over the distal segment of the tibialis
anterior and soleus (PF group) or vastus lateralis and biceps
femoris (quadriceps group). A ground electrode was secured
superficially to the head of the tibia. Thorough skin prepara-
tion for all electrodes included sanding of the skin around the
designated areas followed by cleansing with an isopropyl
alcohol swab. Agonist and antagonist EMG activities were
analyzed during maximal voluntary contractions (MVCs).
EMG activity was amplified (Isolation Head Stage 830 ampli-
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fier, Biomedical 830 amplifier CWA, Ardmore PA), filtered
(10–1,000 Hz), monitored on oscilloscope, and stored on
computer. The computer software program rectified and
integrated the EMG signal (iEMG) over a 500-ms period
during a MVC. iEMG activity was normalized to prefatigue
values for analysis. M-wave amplitudes elicited by the twitch
were measured under the same conditions before MVCs, both
pre- and postfatigue.
Pre- and postfatigue measurements. Peak twitch torques
were evoked with electrodes connected to a high-voltage
stimulator (model DS7H1, Digitimer Stimulator). The amper-
age (10 mA to 1 A) and duration (50–100 µs) of a 400-V
rectangular pulse were progressively increased until a maxi-
mum twitch torque was achieved in the PF. Dependent on the
mass of an individual’s quadriceps, voltage was increased
until either a plateau in the twitch torque was obtained or the
stimulator reached maximum output. Nine of 16 quadriceps
subjects achieved maximal twitch torque. Prefatigue, the
average of three trials was used to measure twitch amplitude,
time to peak twitch torque (TPT), and half relaxation time
(RT1/2).
The ITT was administered with a series of 3-s-duration
MVCs (3 trials). Three doublets (2 twitches with a 10-ms
interval) interspersed at 900-ms intervals were evoked and
superimposed on the voluntary contractions to obtain an
average response (Fig. 1). Superimposed doublets were uti-
lized in an attempt to ensure a large signal-to-noise ratio. Two
potentiated doublets were also recorded at 1-s intervals afterTable 1. Subject characteristics
Group
Height,
cm
Weight,
kg
Age,
yr
Gender
M F
75% PF 163.467.1 71.7613.0 25.363.4 5 3
50% PF 160.9610.2 70.1612.1 21.769.6 4 4
25% quadriceps 166.866.9 76.8617.4 24.665.2 5 3
50% quadriceps 162.868.9 69.4621.6 22.467.4 4 4
Values are means 6 SD for 8 subjects in each group. PF, plantar
flexors; M, male; F, female.
Fig. 1. Top: prefatigue (left) and postfatigue (right) of long-duration fatigue (LDF) protocol [25% maximum
voluntary force contraction (MVC) of quadriceps]. Bottom: prefatigue (left) and postfatigue (right) of short-duration
fatigue (SDF) protocol (50% MVC of quadriceps). Increase in interpolated twitch ratio postfatigue was greater in
subject after LDF protocol than after SDF protocol. Also illustrated is greater drop in MVC immediately after
fatigue observed in LDF protocol. Twitch preceding interpolated twitch technique postfatigue was compared with
unpotentiated evoked resting twitch (twitches generated at rest before any voluntary muscle contractions; data not
shown). y-Axes are in volts. Pre- and postfatigue gains are the same.
1655EFFECTS OF DURATION AND MUSCLE TYPE ON FATIGUE
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the voluntary contractions. Torque signals were sent through
both low- and high-gain amplifiers. The resident software
program offset the gained superimposed signal, 100ms before
each stimulation, for improved resolution. A ratio was calcu-
lated that compared the amplitudes of the superimposed
doublets with the potentiated doublet, representing muscle
fibers that were not voluntarily activated. The percentage of
muscle fibers activated was estimated by subtracting the
ratio from a value of 1 and multiplying by 100 to represent an
index of muscle activation during a voluntary contraction.
Fatigue. After voluntary and evoked testing, the subjects
proceeded with the fatigue test. Each of the four groups were
subjected to a different contraction intensity. The two quadri-
ceps groups performed voluntary contractions at 50 or 25%
MVC while the PF groups performed voluntary contractions
at 50 or 75% MVC. Preliminary work indicated that quadri-
ceps contractions performed at 25% of MVC led to a similar
number of contractions as when work was performed with the
PF at 50% MVC. In addition, the time to fatigue for the
quadriceps at 50%MVCwas similar to the fatigue duration of
the PF at 75% MVC. However, the number of contractions to
fatigue was less in the latter (50% quadriceps, 75% PF) than
in the former (25% quadriceps, 50% PF), leading to what will
be referred to as short- (SDF) and long-duration fatigue
(LDF) protocols. In all protocols, the subject’s contraction
intensity was gradually increased for 3 s until the desired
force was attained. This intensity was maintained for 10 s,
followed by a 3-s gradual decrease to a resting state. The
sequence was resumed after a 4-s rest period. Contraction
cycles (work-to-rest ratio of 16 min 4 s) continued until the
effects of fatigue disrupted the subject’s ability to maintain
the desired force for the 10-s period. Voluntary and evoked
properties were monitored immediately postfatigue and after
30 s and 1, 2, 5, and 10 min of recovery.
Statistical analyses. The effect of fatigue duration and
muscle type on voluntary and evoked twitch properties were
analyzed by using a three-way analysis of variance with
repeated measures on the third factor. The three factors (2 3
2 3 7) included muscle type (quadriceps and PF), fatigue
duration (long and short), and testing period (prefatigue,
postfatigue, and recovery periods of 30 s and 1, 2, 5, and 10
min). F-ratios were considered significant at P , 0.05. If
significant interactions were present, a Tukey post hoc test
was conducted. Descriptive statistics include means 6 SD.
Figures 1–7 include means 6 SE.
RESULTS
Fatigue. When contracting at the same intensity
(50% MVC), the quadriceps (4 min 4 s 6 1 min 2 s)
fatigued more rapidly than did the PF (19 min 33 s 6 8
min 0 s). However, there were no significant differences
in the time to fatigue between the 50% PF (19 min 33 s)
and 25% quadriceps (19 min 27 s 6 3 min 8 s) groups
(LDF) or between the 75% PF (4 min 30 s 6 1 min 44 s)
and 50% quadriceps (4 min 4 s) groups (SDF). The LDF
group (50% PF and 25% quadriceps) had a significantly
(P , 0.0001) greater number of contractions to fatigue
than did the SDF group (75% PF and 50% quadriceps).
Immediately after the fatigue protocol, the LDF group
had a significantly (P5 0.03) greater drop inMVC force
than did the SDF group (40% LDF vs. 30.9% SDF).
MVC recovered to the same extent in the subsequent 10
min of recovery in both the LDF and SDF groups,
although significant differences from prefatigue were
still observed (Fig. 2).
Muscle activation. Before the fatigue test, full muscle
activation was achieved in 11 of 16 PF subjects and 5 of
16 quadriceps subjects. Changes in activation with
fatigue were influenced by the duration of the fatigue
protocol and not by muscle type (Fig. 1). When aver-
aged over the entire recovery period, the index of
muscle activation decreased significantly (P 5 0.02)
more in the LDF (12 6 7.5%) than in the SDF protocol
(5.8 6 4.5%; Fig. 3).
In contrast, changes in M wave after fatigue were
influenced by both fatigue duration (Fig. 4; P 5 0.003)
and muscle type (P 5 0.002). LDF protocols diminished
M-wave amplitudes by 14.7 6 15.5%, contrasting with
the 15.7 6 25.6 potentiation with SDF (Fig. 5). Muscle
type differences were demonstrated by the average
Fig. 2. Mean percent drop in MVC after fatigue in LDF (j) and SDF
(n) groups. Pre, prefatigue; Post, postfatigue. Vertical bars, SE.
Vertical arrow, significant difference between groups (P , 0.05);
horizontal arrow, significant difference from prefatigue values for
both groups (P , 0.05).
Fig. 3. Mean percent drop in index of muscle activation over prefat-
igue, postfatigue, and recovery testing periods in LDF (j) and SDF
(n) groups. Bars, SE. Vertical arrows, significant differences between
groups (P , 0.05); horizontal arrow, significant difference from
prefatigue values for both groups (P , 0.05).
1656 EFFECTS OF DURATION AND MUSCLE TYPE ON FATIGUE
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16.7 6 15.5% potentiation of the quadriceps M waves,
contrasting with the 15.7 6 25.5% reduction in PF M
waves throughout the recovery period (Fig. 6).
The most important factor underlying the changes in
maximum iEMG after fatigue was not as clear cut.
Irrespective of fatigue duration, all PF iEMG activity
significantly decreased after fatigue to a similar extent
(50% PF: 19.36 25.2%; 75% PF: 26.36 8.7%).Although
the LDF quadriceps group (25% MVC: 30.4 6 17%)
experienced a corresponding iEMG decrease, the SDF
quadriceps group (50% MVC: 3.7 6 1.9%) exhibited no
significant change in iEMG after fatigue.
To ensure that changes in soleus iEMG represented
the activity of the triceps surae, gastrocnemius iEMG
activity was calculated after a 50% MVC fatiguing
protocol of the PF in five subjects. Medial and lateral
gastrocnemius iEMG activity had an average decrease
over the entire recovery period of 22.9 6 8.1 and 20.7 6
11.6%, respectively.
Evoked twitch contractile properties.Changes in peak
twitch torque with fatigue were dependent on muscle
type and not fatigue duration (Fig. 7). Quadriceps
twitch torque had an insignificant (P 5 0.34) average
decrease of 14.1 6 2.3%, contrasting with the 16.1 6
2.6% potentiation of the PF (P 5 0.004) during the
recovery period (Fig. 6). TPT was affected by both
muscle type and fatigue duration. The lack of change in
PF TPT contrasted with the significant prolongation
(P 5 0.02) of the quadriceps TPT(15.3 6 3.2%) over the
entire recovery period (Fig. 6). Fatigue-duration effects
were exhibited by the subjects in the SDF protocol, who
experienced significantly (P5 0.008) longer TPT postfa-
tigue and at 30 s of recovery than did subjects in the
LDF protocol (Fig. 5).
In contrast, fatigue duration was the only significant
(P 5 0.0007) factor affecting RT1/2. The RT1/2 of the LDF
group was shortened 16.8 6 12.2 compared with the
9.7 6 2.5% increase in the RT1/2 of the SDF group
during the recovery period (Fig. 5).
Reliability. Intraclass correlation coefficients were
used to determine the test-retest reliability of the
variables. Very high correlation coefficients (.0.9) were
Fig. 4. Quadriceps M wave of 2 subjects pre- and postfatigue. Decrease in M wave after LDF protocol (25% MVC)
contrasted with increase observed after SDF protocol (50%MVC). See Fig. 1 legend for more details.
1657EFFECTS OF DURATION AND MUSCLE TYPE ON FATIGUE
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Fig. 5. Mean percent change in time to peak twitch torque (TPT; top),
half-relaxation time (RT1/2;middle), and M-wave amplitude (bottom)
after fatigue in LDF (j) and SDF (n) groups. Vertical bars, SE.
Vertical arrows, significant differences between groups, (P , 0.05);
horizontal arrow, significant difference from prefatigue values for
both groups (P , 0.05).
Fig. 6. Mean percent change in peak twitch (top), TPT (middle), and
M-wave amplitude (bottom) after fatigue in quadriceps (hexagons)
and plantar flexors (stars). Vertical bars, SE. Vertical arrows, signifi-
cant differences between muscles (P , 0.05); horizontal arrows,
significant differences from prefatigue values for both muscles
(P , 0.05).
1658 EFFECTS OF DURATION AND MUSCLE TYPE ON FATIGUE
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established for the index of muscle activation, MVC,
potentiated doublet, TPT, and RT1/2. Moderate to high
correlation coefficients were found with PF twitch
torque (0.74) and quadriceps twitch torque (0.54).
DISCUSSION
One of the most important findings of this study was
that fatigue-related changes of specific voluntary and
evoked contractile properties were influenced by differ-
ent factors. Fatigue duration exerted its greatest effect
on muscle activation and RT1/2. However, the effect of
fatigue on twitch torque was primarily determined by
muscle type. Fatigue-related changes inM-wave ampli-
tude and TPT were affected by both duration and
muscle type (Table 2).
Muscle activation. Increased time to fatigue resulted
in greater decreases in muscle activation. The 12%
decrease in the index of muscle activation after fatigue
in the LDF group was significantly greater than the
5.8% decrease in the SDF group. The soleus has been
reported to be more susceptible to central (neural)
fatigue than the quadriceps (4). This could suggest that
fatigue-induced muscle inactivation was related to the
specific muscle. It was shown in this study, however,
that the muscle type was incidental to the amount of
muscle inactivation. Alterations in contraction inten-
sity allowed the 25% quadriceps and 50% PF groups to
contract longer than the 50% quadriceps and 75% PF
Fig. 7. Top: plantar flexors twitches before (left) and after (right) fatigue in 1 subject. Bottom: potentiation of plantar
flexors twitches contrasted with marked depression in quadriceps twitches. See Fig. 1 legend for more details.
Table 2. Factors (muscle type or fatigue duration)
affecting selected voluntary and evoked
contractile properties
Muscle Type Fatigue Duration
iEMG Yes Yes (?)
Twitch torque Yes No
TPT Yes Yes
M-wave amplitude Yes Yes
RT‰ No Yes
Muscle activation No Yes
iEMG, integrated EMG activity; TPT, time to peak twitch torque;
M wave, compound muscle action potential; RT‰, half relaxation
time.
1659EFFECTS OF DURATION AND MUSCLE TYPE ON FATIGUE
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groups, resulting in greater inactivation in the LDF
groups independent of the muscle utilized.
M-wave amplitudes in this study decreased with
LDF but increased with SDF. Bigland-Ritchie et al. (7),
who used a 60-s MVC fatigue protocol of the adductor
pollicis, did not find any change in M-wave amplitude,
suggesting muscle membrane transmission failure was
not a component of high-intensity, short-term fatigue.
Other studies using short-duration voluntary (4, 22) or
evoked (18) fatigue protocols have not shown decreases
in M-wave amplitude. Decline in M-wave amplitude
has been reported in the abductor pollicis, after 90–100
s of fatiguing MVCs (3). Fuglevand et al. (17) illus-
trated greater declines in M-wave amplitude with
lower intensity (i.e., longer duration) contractions and
concluded that when force is sustained at a submaxi-
mal value, impairment in muscle membrane propaga-
tion may occur. Decline in M-wave amplitude with LDF
was evident in our study as well, indicating an impair-
ment of themusclemembrane excitability or neuromus-
cular propagation. The 14.7% depression of M-wave
amplitude of the LDF group contrasted with the 15.7%
potentiation of the SDF group.
The fact that the M wave potentiated during SDF
would suggest that muscle membrane propagation
failure is not the primary determinant of muscle fa-
tigue with SDF. Potentiation of the M wave during the
first 30 s of tetanic fatiguing contractions has also been
observed, suggesting an increased excitability of the
muscle fibers (15). M-wave potentiation may signify
that presynaptic and/or end-plate potentials are facili-
tated possibly by a reduction in the dispersion of fiber
action potentials (12). The increase in M-wave ampli-
tude could support Buchthal and Madsen’s (9) report of
increased synchronization with fatiguing contractions.
Alterations in M-wave amplitude were dependent
not only on fatigue duration but also on muscle type.
Overall, quadriceps M waves were potentiated 16.7%
while PF M waves were depressed 15.7%. Milner-
Brown and Miller (26) concluded that the impairment
of membrane propagation depends both on the duration
and degree of fatigue and on the intrinsic properties of
the individual muscle. Pagala et al. (28) reported
greater decreases in the action potentials of the exten-
sor digitorum longus and diaphragm than with the
soleus while Moritani et al. (27) found similar results
when comparing the gastrocnemius and soleus.
The unchanged iEMG activity of the 50% quadriceps
group (3.7%) contrasted with significant decreases for
both PF and 25% quadriceps groups. Because the
extent of muscle activation and compound muscle
action potential (M wave) both contribute to the EMG
signal, the differing response of the 50% quadriceps
group might suggest that the EMG is also affected by
both duration and muscle type.
Evoked twitch contractile properties. Changes in
twitch torque were muscle dependent. Quadriceps
twitch torque had an insignificant 14.1% decrease, in
contrast to the 16.1% potentiation of PF. Conversely,
alterations in TPT were both muscle and fatigue dura-
tion dependent. Quadriceps TPT was prolonged 15.3%,
whereas PF TPT was not significantly changed. The
prolongation of quadriceps TPT may help to explain
why the decreased quadriceps twitch torque did not
achieve significance. Muscle dependence was also re-
ported by Hatcher and Luff (19), who found contrasting
results when comparing the heterogenous flexor digi-
torum longus (FDL) and slow-twitch soleus muscles of
the cat. In contrast to the significant changes in the
FDL, the soleus had smaller decreases in tetanicten-
sion and no change in maximum shortening velocity.
The decline in quadriceps twitch torque and prolonga-
tion of TPT in the present study may imply an impair-
ment in excitation-contraction (E-C) coupling (13, 30).
The quadriceps M-wave potentiation would argue
against failure in membrane propagation, suggesting
the impairment was related more to the sarcoplasmic
reticulum’s release of Ca21 and/or cross-bridge kinetics.
Potentiation of PF twitch torque and a lack of change in
PF TPT in conjunction with decreases in M-wave
amplitude would reinforce the hypothesis that impair-
ments in PF-evoked properties can be mainly attrib-
uted to impairments of membrane potentials.
Fatigue duration was themajor factor affecting RT1/2,
with subjects in the LDF group experiencing a 16.8%
decrease, in contrast with a 9.7% increase in RT1/2 of
subjects in the SDF group. Alterations in twitch torque
or TPT did not correspond to changes in RT1/2. The
divergence of TPT and RT1/2 has also been reported by
Viitasalo and Komi (30), who found differing recovery
profiles. Bigland-Ritchie et al. (9) demonstrated a pro-
longation in RT1/2 contrasted with a lack of change in
TPT. Impairment in E-C coupling affecting Ca21 release
(TPT) may not automatically coincide with a hindrance
of Ca21 sequestering (RT1/2). The sequestration of Ca21
is an active process involvingATP (8) and thus would be
affected by alterations in the muscle metabolic milieu.
Resynthesis of ATP in glycolysis has been suggested to
be inhibited by low intramuscular pH (29) and thus
may affect RT1/2 more with the higher intensity contrac-
tions of the SDF. The release of Ca21, however, is not an
active process (8). The differing effects of metabolism
may explain the lack of correlation between changes in
RT1/2 and the other evoked contractile properties of
twitch torque and TPT.
Summary. In summary, this study found duration-
induced impairments in muscle activation and RT1/2,
muscle-dependent decreases in peak twitch torque and
iEMG, and effects by both factors in M-wave amplitude
and TPT. The decrease in LDF muscle activation and
M-wave amplitude indicated that impairment inmuscle
activation and membrane action potentials contributed
to fatiguewith long-duration contractions. The potentia-
tion of SDFMwaves decreased the possibility of muscle
membrane impairments. Potentiation of PF twitch
torque and a lack of change in PF TPT in conjunction
with decreases in M-wave amplitude would suggest
impairments of PF membrane potentials. Fatigue-
related decreases in quadriceps twitch torque and
prolongation of TPT may be associated with disrup-
tions in E-C coupling. Impairments in RT1/2 were
1660 EFFECTS OF DURATION AND MUSCLE TYPE ON FATIGUE
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directly affected by fatigue duration and thus possibly
related to muscle metabolic changes.
Present address and address for reprint requests: D. G. Behm,
School of Physical Education and Athletics, Memorial Univ. of
Newfoundland, St. John’s, Newfoundland, CanadaA1C 5S7.
Received 20 October 1995; accepted in final form 6 January 1997.
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1661EFFECTS OF DURATION AND MUSCLE TYPE ON FATIGUE
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